Lachrymatory agents are commonly used for riot control. Their use as chemical warfare agents is prohibited by various international treaties. During World War I, increasingly toxic lachrymatory agents were used.

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Tear gas works by irritating mucous membranes in the eyes, nose, mouth and lungs, and causes crying, sneezing, coughing, difficulty breathing, pain in the eyes, and temporary blindness. With CS gas, symptoms of irritation typically appear after 20–60 seconds of exposure[1] and commonly resolve within 30 minutes of leaving (or being removed from) the area.[2] With pepper spray (also called "oleoresin capsicum", capsaicinoid or OC gas), the onset of symptoms, including loss of motor control, is almost immediate.[2] There can be considerable variation in tolerance and response, according to the National Research Council (US) Committee on Toxicology.[3]

The California Poison Control System analyzed 3,671 reports of pepper spray injuries between 2002 and 2011.[4] Severe symptoms requiring medical evaluation were found in 6.8% of people, with the most severe injuries to the eyes (54%), respiratory system (32%) and skin (18%). The most severe injuries occurred in law enforcement training, intentionally incapacitating people, and law enforcement (whether of individuals or crowd control).[4]

Lachrymators are thought to act by attacking sulfhydryl functional groups in enzymes. One of the most probable protein targets is the TRPA1 ion channel that is expressed in sensory nerves (trigeminal nerve) of the eyes, nose, mouth and lungs.

As with all non-lethal, or less-than-lethal weapons, there is some risk of serious permanent injury or death when tear gas is used.[5][6] This includes risks from being hit by tear gas cartridges, which include severe bruising, loss of eyesight, skull fracture, and even death.[7] A case of serious vascular injury from tear gas shells has also been reported from Iran, with high rates of associated nerve injury (44%) and amputation (17%),[8] as well as instances of head injuries in young people.[9]

Use of tear gas in warfare (as all other chemical weapons) is prohibited by various international treaties[13] that most states have signed. Police and private self-defense use is not banned in the same manner. Armed forces can legally use tear gas for drills (practicing with gas masks) and for riot control. First used in 1914, xylyl bromide was a popular tearing agent since it was easily brewed.

Certain lachrymatory agents are often used by police to force compliance, most notably tear gas.[6] In some countries (e.g., Finland, Australia, and the United States), another common substance is mace. The self-defense weapon form of mace is based on pepper spray, and comes in small spray cans, and versions including CS are manufactured for police use.[15] Xylyl bromide, CN and CS are the oldest of these agents, and CS is the most widely used. CN has the most recorded toxicity.[1]

Typical manufacturer warnings on tear gas cartridges state "Danger: Do not fire directly at person(s). Severe injury or death may result."[16] Such warnings are not necessarily respected, and in some countries, disrespecting these warnings is routine. In the 2013 protests in Turkey, there were hundreds of injuries among protesters targeted with tear gas projectiles.[citation needed] In the Israeli-occupied territories, Israeli soldiers have been routinely documented by Israeli human rights group in firing direct tear gas canisters at activists, some of which resulted in fatalities.[17][18]

However, tear gas guns do not have a manual setting to adjust the range of fire. The only way to adjust the projectile's range is to aim towards the ground at the correct angle. Incorrect aim will send the capsules away from the targets, causing risk for non-targets instead.[19] For example, this occurred during the 2013 protests in Brazil.

Tear gas grenade returned to soldiers using sling during weekly protest in Ni'lin, July 2014

There is no effective antidote to common tear gases.[1] Getting clear of gas and into fresh air is the first line of action.[1] Once a person has been exposed, there is a variety of methods to remove as much chemical possible and relieve symptoms.[1] The standard first aid for burning solutions in the eye is irrigation (spraying or flushing out) with water,[1][21] and some evidence suggests that diphoterine[22] solution, a first aid product for chemical sprays, may help with ocular burns or chemicals in the eye.[21][23]

Activists in the United States, the Czech Republic, Venezuela and Turkey have reported using antacid solutions such as Maalox diluted with water as a home remedy for tear gas attacks.[24][25][26] Vinegar, vaseline, milk and lemon juice solutions have also been used by activists.[27][28][29][30] It is unclear how effective these remedies are. In particular, vinegar itself can burn the eyes and prolonged inhalation can also irritate the airways.[31]